Solar Rice Drying In Aceh
SOLAR RICE DRYING IN ACEW
Solar Energy Research a t Syiah Kuala University,
University of Kentucky, U.S.A.
ABSTRACT
This paper presents the results and observations to date from the solar energy program at the
University of Syiah Kuala in Aceh. The solar energy program is starting its second year of projects
which include the following: development and construction of a solar water distillation unit, the
recording of solar radiation intensity data, the investigation of traditional methods of rice drying
and the construction of a one ton solar rice drier, based on an A.I.T. design.
A summary of some of the water distillation project results helps set the background for an
examination of solar radiation energy in Aceh and possible new solar rice drying methods. Results
and observations from a study on the traditional rice drying methods of bamboo matt and concrete
surface drying are given. And, data from a sociological study on rice drying and storage in Aceh are
discussed.
A brief description of the A.I.T. solar rice drier is given; and its advantages, place in the
UNSYIAH solar energy research program and possible future uses in Aceh are discussed.
Comments concerning the long range integration of large volume solar rice drying into the
Acehenese culture are also made.
SOLAR ENERGY RESEARCH A T UNSYIAH
In 1 9 8 2 , the University of Syiah Kuala a t Banda Aceh contacted the BKSWUEA educational development project about an instructor t o help teach and
develop a program in the area of solar energy. The idea was t o develop not only
a course in solar energy at the under-graduate level that would help train future
teachers and researchers, but at the same time, begin some low level solar
energy experiments that could be used by extension programs in the province
of Aceh. I was applying for a position w i t h the BKS-WUEA project a t that time
and arrived at my post i n January of 1 9 8 3 .
The BKS-WUEA project is administered under a contract from the
American Embassy by the University of Kentucky in the U.S.A., w i t h a focus
on developing the college of agriculture in eleven universities in Sumatra and
Kalirnantan. Because of this agricultural focus, w e were concerned that the
research done a t UNSYIAH not only be solar in nature, but also related to the
agricultural or rural development problems in Aceh. Several areas of study
were discussed, of which, the development of a solar water distillation - salt
production unit and a solar rice drying unit were judged as most suitable.
In August of 1 9 8 3 , construction was begun on a 1 6 m2 water distillation
unit which was completed in November 1 9 8 3 . During this past year, data on
the distillation of low salt concentration well water was taken, which is
pointing the way towards a more inexpensive design for village use and
development. Future experiments are also being laid for the testing of ocean
water distillation, which we hope will lead t o ways of greatly reducing the
energy requirements for salt production in Aceh.
During the past school year we have also considered ways t o begin our
research in solar rice drying. One student expressed interest in studying the
traditional methods of rice drying in Aceh for his senior design project, and at
this time, is finishing a sociological and heat balance study of rice drying on
bamboo matt and concrete. We are following up on this background work with
the construction of a one ton solar rice dryer that was developed at A.I.T. in
Bangkok. Construction has begun and should be finished sometime in September. Basic performance in rice drying will then be tested and compared t o
the results published by A.I.T. and if successful, additional tests on clove
drying will be done next year.
In this report, I will present some of the results from the various aspects of
our budding solar energy program, and then proceed t o describe some of
our ideas for future research in solar rice drying for Aceh.
TRADITIONAL RICE DRYING METHODS IN ACEH
I n Aceh, most families own less than a hectare of land and harvest an
average of 2 metric tons per hectare, per harvest. Harvest times come three
times per year, with the first rice season running from August t o November, t o
be followed by an alternate crop from December t o March and returning t o rice
again from April to July. Turn around time between harvesting and planting is
around one month, during which all of the post-harvest and soil preparation
work needs t o be dorte.
Harvesting is done by hand, using a sycle, as is common in Indonesia.
After the grain is cut, it is piled on the side of the sawah and left t o dry for
approximately 3 to 6 days before it undergoes threshing. The threshing itself is
done by hand and is accomplished by simply striking the rice stalks against
something. Afterwards, the grain is cleaned by pouring the grain from a 1 %
meter height t o the ground. The wind takes away most of the chaff and the
rest is cleaned by hand. The moisture content at this point was measured and
found t o average around 22 %.
A t this point in the process, people generally set aside a small portion for
reseeding and sell the rest to a cooperative. Sacks of approximately 80 kg
capacity are used for this and most family complexes will have several of these
sacks stored in a corner. For some households, however, an alternative
method of storage is used. This consists of a container of about 2 cubic meters
volume constructed of wood. On the bottom is a bamboo matt and along its
four sides are pieces of wood or oil-palm branches. These containers many
times have their o w n platform and roof and stand by themselves at a short
distance from the house, yet within the family compound. Maximum possible
storage time is generally 6 months at which time, the rice begins t o be ruined
by mold, insects and deterioration.
Whenever a family needs t o use some of their rice, they will remove it from
storage and then dry it on a bamboo matt for one day before having it milled.
The bamboo matt varies in size but averages approximately 2 % m x 3 % m for
8.75 rn2 of drying area. This matt is placed near the house somewhere, with
either soil, grass or concrete directly underneath it. The rice has an average
initial moisture content of 2 1 % at this time and is placed on top of the matt t o
a height of 0.5-1 cm. It is then dried for 6 - 7 hours during which time the
rice is turned over twice. Moisture content from samples taken at the end of
the day ran between 1 1 % - 13 %.
Larger cooperatives use a concrete surface located near their milling and
storage unit. Rice previously bought from the farmers a t 2 2 % moisture
content is stored in sacks a t the mill until there is time t o mill it. One day before
the rice is t o be milled, it will be layed out on the concrete t o dry. Average
moisture content of this grain is between 1 9 - 2 1 % and the rice is placed on
top of the concrete t o a depth of 4 - 5 cm. Drying goes on for about 7 hours
and repeatedly the grain is turned over with a flat wooden paddle. The
moisture content at the end of the drying process was found t o be between
13.5-1 5%.
The size of drying areas runs between 5 0 m2 t o 1 5 0 m2 and often the
surface of the concrete is not flat but shaped like a sinusoidal curve w i t h about
1 meter between the high points, as given in the following figure. The
advantages of this system stem not from its drying ability, but from the
advantage of the operator being able t o collect the rice on the high points
during a sudden rain storm. The operator will cover the rice w i t h plastic if the
rain is light and leave the low areas free for rain water runoff. When the rain
Fig. 1.
Sinusoidal Concrete Surface.
passes, the rice is re-distributed over the surface once more and the drying
continues.
Rain is a real problem in Aceh in that, even though it rains more i n the
months November t o January, there are no set seasons and the possibility of
rain is a daily reality. If the rain is too hard, the operator is forced t o p u t the rice
back into its sacks and continue drying the next day. This in turn brings u p one
of the major disadvantages of this type of drying method - the occurrence of
losses due t o handeling. It was not the purpose of our research t o study the
impact handeling losses have on this drying method, but from other studies,
w e know that they range from 5 t o 2 0 % .
HEAT TRANSFER CONSIDERATIONS OF TRADITIONAL RICE DRYING
Six one-day tests were made t o study the temperature variations within a
layer of rice on bamboo matts and concrete. Testing was done out-of-doors on
a drying site in Aceh. Because of the testing conditions, the data taken is not
steady state in nature and makes it difficult t o address in depth any theoretical
heat transfer concepts involved. However, it is possible t o look a t the overall
heat baldnce and general heat f l o w mechanisms.
Thermometers were placed at various positions around and i n the rice in
the following manner. ( 1 ), A thermometer was placed in the ground below the
bamboo matt and in the concrete t o measure the heat flow t o the ground
beneath the rice. ( 2 ) , A thermometer was placed on the matt or concrete
surface t o measure the temperature at the lowest level of the rice. ( 3 ) , A
thermometer was placed just under the surface of the rice so that the
temperature of the top layer of rice could be measured. (4), The w e t bulb and
dry bulb temperatures of the air, about 10 c m above the rice, was taken.
Data on solar intensity, wind velocity and barometric pressure were taken
every 30 minutes while rice samples were taken each hour t o measure the
change in moisture content.
Bamboo Matt
The bamboo matt, as is typically done, was placed on top of grass instead
of open ground. An air space of 2 c m was thus formed between the matt and
ground. Wind velocity during all of the tests was fairly light a t a velocity of
1 % mlsec w i t h an occasional period of t w o hours when wind velocity reached
4 mlsec. Because of this, the air space below the matt tended t o act as a layer
of insulation.
The ground temperature below the matt was found t o vary w i t h ambient
temperature, (dry bulb measurement), above the matt, and differences
between these t w o temperatures were nearly always less than 2 OC. Ambient
air temperatures ranged from 29OC at 9 : 30 t o 36OC a t 1 2 : 30 w i t h tempe-
rature differences in the morning showing the' ambient temperature 1 -2OC
higher than the ground temperatures, while afternoon temperatures show the
ground temperatures running 1 -2 OC higher than the ambient temperature.
M a t t temperatures were always higher than the ground temperatures,
ranging from 35OC t o 46OC on a clear day and up t o 42OC on an overcast
day. Temperature differences between the ground and matt were 7-8O6,
with an occasional period of 12O6, as opposed t o the 1 -2OC difference
between the ground and ambient temperatures. This fact adds t o our previous
observation of an insulating dead air space below the matt.
Temperatures at the top of the rice layer ranged from 36OC t o 49.5OC or
from 1-4OC higher the matt temperature. Differences between the top of the
rice and ambient temperatures ran between 7 OC in the morning t o 10- 13OC
by mid-day.
Moisture content started at 22% each time and dropped t o 17% by
11 : 3 0 on an average day and t o 14% by 11 : 30 on clearer days. A better
indicator was found t o be the top and bottom temperatures of the rice layer,
which were always 40° C and 39 6 respectively when the rice reached 17%
moisture content. Thus, with a 3-4OC temperature rise, the rice will drop 5
percentage points of moisture. From this point on, the drying rate changed
from 0.9- 1.2 percentage points per hour until 13:00, when the rate changed
t o 0.6-0.8 percentage points per hour. This lasted until either the sun set or
the 1 1 .5% moisture level. A t this 1 1 .5% moisture level, the rate of drying
changed once more t o 0.2 percentage points per hour. Final moisture contents
for the 6 tests on a 1 c m deep layer ran from 1 1 - 1 3.8%.
Concrete Surface
The concrete slab used in these tests was 8 c m thick and flat. No paint
was applied t o the surface. For this test a hole was drilled into the concrete
and a thermometer stuck tip first into the concrete. The other grain thermometers were again placed at the bottom and top of the rice layer which was
4 c m thick for each of the tests.
The concrete temperature maintained a fairly close range of 38.5-41 OC
and was found t o be always higher in temperature than the lower level of rice.
Generally it was 1 % - 2 C higher in temperature, though a t times it would
have the same value in the middle of the day. The temperature of both the
concrete and lower level of rice varied in a similar manner as the ambient
temperature, but within a closer range. The ambient temperatures were the
same as those in the matt drying tests and varied from 29 OC t o 36 OC within a
day's cycle.
Because of the high conductivity of the stone compared t o the loosely
packed rice, heat conduction away from the rice t o the ground does happen.
However, a basic heat balance across the concrete slab shows that heat loss
by conduction amounts t o less than 1 % of the incoming solar energy.
The top of the grain temperature varied from 34-46OC or 3-6OC higher
than the lower part of the rice layer. Temperature differences between the top
of the grain and the ambient air ran from 5OC during the morning to 13OC
around mid-day.
Moisture content started at 2 2 % each time and dropped t o 1 7 % by
1 4 : 0 0 on an average day (though on a clear day it was accomplished by
1 2 : 30). A better indicator here for the 1 7 % moisture point was a 45OC top
temperature and a 39OC lower level temperature. Drying rates do not appear
to be constant and vary between 0.7-1.2 percentage points per hour until
the grain reaches 14.5% moisture. A t this point, the rate drops t o 0.6-0.8
percentage points per hour. However, it usually takes the entire day to reach
14.5% moisture, and it is difficult t o say whether or not an actual change in
drying rate occurs or whether the solar radiation intensity has simply slacked
off at the day's end. Final moisture contents at the end of the day for the 4 cm
layer ranged from 13.7- 14.9%.
Comparison
Except for the physical layout, the methods are quite similar and only four
characteristics need t o be compared to understand their differences. (1), The
volume of the concrete surface method is four times that of the matt method,
yet, (2), only the temperatures under the rnatt and in the concrete differ
significantly. The third, (3), characteristic is that the matt drying shows all
three textbook stages of drying, (initial, falling and leveling out), while the
concrete surface method operates in only the first t w o stages where most of
the water is lost. Thus the matt drying method shows a greater volume of
water lost per volume of rice. However, the optimum moisture content is
1 3 - 14%, and it is not optimum to dry the rice t o the extent that the matt
method does. In turn, the final moisture content levels of the concrete surface
method need t o be a little lower. This could possibly be accomplished by drying
more rice at one time with the matt method and less rice with the concrete
surface method.
The last, (4fh), characteristic is that of an overall efficiency. If we define
the efficiency as the energy used to vaporize the water, divided by the solar
energy that falls on the rice surface, we can determine the overall performance
of each method. Using this definition, the efficiency for the bamboo matt
method is approximately 8%, while the concrete surface method has an
approximate efficiency of 2 6 % .
If all of the rice had dried at the same performance level as the matt
method, we would have expected the concrete surface method's efficiency to
be 4 x 8 % or 32%. However, because there was a difference in the final
moisture contents of the t w o methods, the concrete surface method had a
lower efficiency. Thus, if the height of the grain on the concrete surface is
decreased, we expect the moisture content to decrease as well. This suggests
that an optimum efficiency for the matt drying method should be approximately 7.5% while 7.5 x 3 = 22.5% is best for the concrete surface
method.
SOLAR WATER DISTILLATION UNIT
About a year ago we built a solar water distillation unit at UNSVIAH, and
though it has littie to do with rice drying, some of the results cast some light on
the concrete surface method. The unit consists of a concrete slab of about
8 em thickness, with short wails built around it. Two opposing walls are sloped
upwards towards- their centers at a 1O0 slope, and support beams placed
across their centers. Glass windows were then placed over the entire unit, and
the concrete was painted black.
Operation is simple. Water is placed in the unit, which vaporizes in the
heat of the sun. The vapor then rises and condenses when it touches the
windows. From there it flows down the bottom side of the window to pipes
which carry it outside of the unit.
General efficiency of the unit is about 25%. Temperatures of the water rise
to approximately 63OC by 3 3 : 0 0 and the concrete temperatures average
1 - 5 O C higher than the water temperature, while the glass temperatures run
1 -5OC cooler. Average output is around 1.75 I/m2/dayor 2 0 literlday for the
unit.
A heat up time is necessary before vapor is produced each day and
generally it is 11 : 00 before the first liter is produced. After 1 1 : 0 0 water is
distilled at the rate of 2 - 3 liters per hour until dark (19 : 00 in Aceh) and
continues on through the night until all of the usable energy in the concrete
slab is exhausted. Approximately 113 - 112 of the unit's output is produced
after the sun has gone down.
Looking only at the concrete, we see that because of the black color alone
the temperature runs about 15O higher than the top surface temperature of
the rice surface mentioned in the traditional methods discussion. Approximately 4 0 % of the usable energy was stored in the concrete to be used after
the sun had set. This is a solar passive design concept that is used by
architects to store heat in a building for winter heating. Here, there is no
winter, but the same concepts should be applicable in future modifications of
the traditional solar rice drying methods in Indonesia.
A.I.T.
SOLAR RICE
DRYER
A t this time we are in the process of constructing a solar rice dryer
designed by A.I.T. in Bangkok. This unit is made chiefly of bamboo poles and
plastic and consists of a solar collector area with an adjoining room containing
a drying table. The following figure shows the basic layout. From points AA to
BB is the solar 'collector (which is nothing more than an earthen area covered
over with plastic). Air heats up in this area and rises naturally under the plastic
to the bottom of the table, at point C. From there it passes through the rice on
the table and up out of the chimney at point E. Rice is placed on top of the table
by opening the doors at D and simply pouring it out evenly. .
Fig. 2.
A.I.T. Solar Rice Dryer.
The solar collection area for the unit is 4.5 m x 7 m, and the drying table
area is 1.5 m x 7 meters. Unit capacity is one ton or 1.6 m3 of rice, and the
average time to dry the rice from 22% moisture t o 13% is 2 days.
We have selected this unit because it fits both into our solar program as a
solar demonstration experiment and because we wanted to demonstrate a rice
dryer of medium capacity. As far as performance is concerned, there is no
advantage to this unit. The traditional concrete surface method takes the same
drying area for one ton of rice as this unit does, and an operator will usually dry
for only one day.
Characteristics of handeling and uniformity of drying, however, are much
more important with cracking duk to dryness greatly reduced and the chance
of re-wetting from a sudden rain storm eliminated. The rice is also up off of the
ground so that extra dirt from the atmosphere can not settle on the rice, thus
improving the cleanliness of the rice.
The cost of the unit, given by A.I.T., is approximately Rp 200,000.which, at this point in time, is too expensive for most farmers. We are
expecting, however, the demand for not only higher quality rice, but higher
quality agricultural products in general, t o increase in the next ten years. Thus
in turn, creating a demand for this unit.
After preliminary studies on rice drying, we will test the unit for clove
drying. This is a high cash crop for richer urban dwellers who own property in
the country. These people should be willing t o not only purchase such a unit
for their cloves, but also be willing t o use it for rice drying on their rural family
property. In this manner, we hope the unit will be accepted in Aceh over a
10- 15 year period.
FUTURE RESEARCH
Future research in solar rice drying will have t o focus on specific aspects
of both solar radiation energy absorption by the rice and heat storage in the
drying system before serious alternative designs can be made. The use of
either a black surface below the rice or a black plastic sheet on top of the rice,
for example, may increase the heat properties by 5- 10% while storage of
heat in the concrete could provide even higher efficiencies.
Some ideas that I personally have are as follows, (1) Painting the concrete
black for concrete surface drying, ( 2 ) Using a strip of black plastic with holes
punctured in it on top of the rice t o increase the amount of energy absorbed
into the rice, (3)Altering the A.I.T. design so as t o use a black concrete
surface as the solar collection area, and (4) Building a water distillation - salt
production unit that could readily convert to a grain dryer. Determining
whether or not an idea is realistic is the function of a research program, and in
the future, we at UNSYlAH hope that some of our ideas can realistically add to
the performance of the solar rice drying done in Aceh.
Solar Energy Research a t Syiah Kuala University,
University of Kentucky, U.S.A.
ABSTRACT
This paper presents the results and observations to date from the solar energy program at the
University of Syiah Kuala in Aceh. The solar energy program is starting its second year of projects
which include the following: development and construction of a solar water distillation unit, the
recording of solar radiation intensity data, the investigation of traditional methods of rice drying
and the construction of a one ton solar rice drier, based on an A.I.T. design.
A summary of some of the water distillation project results helps set the background for an
examination of solar radiation energy in Aceh and possible new solar rice drying methods. Results
and observations from a study on the traditional rice drying methods of bamboo matt and concrete
surface drying are given. And, data from a sociological study on rice drying and storage in Aceh are
discussed.
A brief description of the A.I.T. solar rice drier is given; and its advantages, place in the
UNSYIAH solar energy research program and possible future uses in Aceh are discussed.
Comments concerning the long range integration of large volume solar rice drying into the
Acehenese culture are also made.
SOLAR ENERGY RESEARCH A T UNSYIAH
In 1 9 8 2 , the University of Syiah Kuala a t Banda Aceh contacted the BKSWUEA educational development project about an instructor t o help teach and
develop a program in the area of solar energy. The idea was t o develop not only
a course in solar energy at the under-graduate level that would help train future
teachers and researchers, but at the same time, begin some low level solar
energy experiments that could be used by extension programs in the province
of Aceh. I was applying for a position w i t h the BKS-WUEA project a t that time
and arrived at my post i n January of 1 9 8 3 .
The BKS-WUEA project is administered under a contract from the
American Embassy by the University of Kentucky in the U.S.A., w i t h a focus
on developing the college of agriculture in eleven universities in Sumatra and
Kalirnantan. Because of this agricultural focus, w e were concerned that the
research done a t UNSYIAH not only be solar in nature, but also related to the
agricultural or rural development problems in Aceh. Several areas of study
were discussed, of which, the development of a solar water distillation - salt
production unit and a solar rice drying unit were judged as most suitable.
In August of 1 9 8 3 , construction was begun on a 1 6 m2 water distillation
unit which was completed in November 1 9 8 3 . During this past year, data on
the distillation of low salt concentration well water was taken, which is
pointing the way towards a more inexpensive design for village use and
development. Future experiments are also being laid for the testing of ocean
water distillation, which we hope will lead t o ways of greatly reducing the
energy requirements for salt production in Aceh.
During the past school year we have also considered ways t o begin our
research in solar rice drying. One student expressed interest in studying the
traditional methods of rice drying in Aceh for his senior design project, and at
this time, is finishing a sociological and heat balance study of rice drying on
bamboo matt and concrete. We are following up on this background work with
the construction of a one ton solar rice dryer that was developed at A.I.T. in
Bangkok. Construction has begun and should be finished sometime in September. Basic performance in rice drying will then be tested and compared t o
the results published by A.I.T. and if successful, additional tests on clove
drying will be done next year.
In this report, I will present some of the results from the various aspects of
our budding solar energy program, and then proceed t o describe some of
our ideas for future research in solar rice drying for Aceh.
TRADITIONAL RICE DRYING METHODS IN ACEH
I n Aceh, most families own less than a hectare of land and harvest an
average of 2 metric tons per hectare, per harvest. Harvest times come three
times per year, with the first rice season running from August t o November, t o
be followed by an alternate crop from December t o March and returning t o rice
again from April to July. Turn around time between harvesting and planting is
around one month, during which all of the post-harvest and soil preparation
work needs t o be dorte.
Harvesting is done by hand, using a sycle, as is common in Indonesia.
After the grain is cut, it is piled on the side of the sawah and left t o dry for
approximately 3 to 6 days before it undergoes threshing. The threshing itself is
done by hand and is accomplished by simply striking the rice stalks against
something. Afterwards, the grain is cleaned by pouring the grain from a 1 %
meter height t o the ground. The wind takes away most of the chaff and the
rest is cleaned by hand. The moisture content at this point was measured and
found t o average around 22 %.
A t this point in the process, people generally set aside a small portion for
reseeding and sell the rest to a cooperative. Sacks of approximately 80 kg
capacity are used for this and most family complexes will have several of these
sacks stored in a corner. For some households, however, an alternative
method of storage is used. This consists of a container of about 2 cubic meters
volume constructed of wood. On the bottom is a bamboo matt and along its
four sides are pieces of wood or oil-palm branches. These containers many
times have their o w n platform and roof and stand by themselves at a short
distance from the house, yet within the family compound. Maximum possible
storage time is generally 6 months at which time, the rice begins t o be ruined
by mold, insects and deterioration.
Whenever a family needs t o use some of their rice, they will remove it from
storage and then dry it on a bamboo matt for one day before having it milled.
The bamboo matt varies in size but averages approximately 2 % m x 3 % m for
8.75 rn2 of drying area. This matt is placed near the house somewhere, with
either soil, grass or concrete directly underneath it. The rice has an average
initial moisture content of 2 1 % at this time and is placed on top of the matt t o
a height of 0.5-1 cm. It is then dried for 6 - 7 hours during which time the
rice is turned over twice. Moisture content from samples taken at the end of
the day ran between 1 1 % - 13 %.
Larger cooperatives use a concrete surface located near their milling and
storage unit. Rice previously bought from the farmers a t 2 2 % moisture
content is stored in sacks a t the mill until there is time t o mill it. One day before
the rice is t o be milled, it will be layed out on the concrete t o dry. Average
moisture content of this grain is between 1 9 - 2 1 % and the rice is placed on
top of the concrete t o a depth of 4 - 5 cm. Drying goes on for about 7 hours
and repeatedly the grain is turned over with a flat wooden paddle. The
moisture content at the end of the drying process was found t o be between
13.5-1 5%.
The size of drying areas runs between 5 0 m2 t o 1 5 0 m2 and often the
surface of the concrete is not flat but shaped like a sinusoidal curve w i t h about
1 meter between the high points, as given in the following figure. The
advantages of this system stem not from its drying ability, but from the
advantage of the operator being able t o collect the rice on the high points
during a sudden rain storm. The operator will cover the rice w i t h plastic if the
rain is light and leave the low areas free for rain water runoff. When the rain
Fig. 1.
Sinusoidal Concrete Surface.
passes, the rice is re-distributed over the surface once more and the drying
continues.
Rain is a real problem in Aceh in that, even though it rains more i n the
months November t o January, there are no set seasons and the possibility of
rain is a daily reality. If the rain is too hard, the operator is forced t o p u t the rice
back into its sacks and continue drying the next day. This in turn brings u p one
of the major disadvantages of this type of drying method - the occurrence of
losses due t o handeling. It was not the purpose of our research t o study the
impact handeling losses have on this drying method, but from other studies,
w e know that they range from 5 t o 2 0 % .
HEAT TRANSFER CONSIDERATIONS OF TRADITIONAL RICE DRYING
Six one-day tests were made t o study the temperature variations within a
layer of rice on bamboo matts and concrete. Testing was done out-of-doors on
a drying site in Aceh. Because of the testing conditions, the data taken is not
steady state in nature and makes it difficult t o address in depth any theoretical
heat transfer concepts involved. However, it is possible t o look a t the overall
heat baldnce and general heat f l o w mechanisms.
Thermometers were placed at various positions around and i n the rice in
the following manner. ( 1 ), A thermometer was placed in the ground below the
bamboo matt and in the concrete t o measure the heat flow t o the ground
beneath the rice. ( 2 ) , A thermometer was placed on the matt or concrete
surface t o measure the temperature at the lowest level of the rice. ( 3 ) , A
thermometer was placed just under the surface of the rice so that the
temperature of the top layer of rice could be measured. (4), The w e t bulb and
dry bulb temperatures of the air, about 10 c m above the rice, was taken.
Data on solar intensity, wind velocity and barometric pressure were taken
every 30 minutes while rice samples were taken each hour t o measure the
change in moisture content.
Bamboo Matt
The bamboo matt, as is typically done, was placed on top of grass instead
of open ground. An air space of 2 c m was thus formed between the matt and
ground. Wind velocity during all of the tests was fairly light a t a velocity of
1 % mlsec w i t h an occasional period of t w o hours when wind velocity reached
4 mlsec. Because of this, the air space below the matt tended t o act as a layer
of insulation.
The ground temperature below the matt was found t o vary w i t h ambient
temperature, (dry bulb measurement), above the matt, and differences
between these t w o temperatures were nearly always less than 2 OC. Ambient
air temperatures ranged from 29OC at 9 : 30 t o 36OC a t 1 2 : 30 w i t h tempe-
rature differences in the morning showing the' ambient temperature 1 -2OC
higher than the ground temperatures, while afternoon temperatures show the
ground temperatures running 1 -2 OC higher than the ambient temperature.
M a t t temperatures were always higher than the ground temperatures,
ranging from 35OC t o 46OC on a clear day and up t o 42OC on an overcast
day. Temperature differences between the ground and matt were 7-8O6,
with an occasional period of 12O6, as opposed t o the 1 -2OC difference
between the ground and ambient temperatures. This fact adds t o our previous
observation of an insulating dead air space below the matt.
Temperatures at the top of the rice layer ranged from 36OC t o 49.5OC or
from 1-4OC higher the matt temperature. Differences between the top of the
rice and ambient temperatures ran between 7 OC in the morning t o 10- 13OC
by mid-day.
Moisture content started at 22% each time and dropped t o 17% by
11 : 3 0 on an average day and t o 14% by 11 : 30 on clearer days. A better
indicator was found t o be the top and bottom temperatures of the rice layer,
which were always 40° C and 39 6 respectively when the rice reached 17%
moisture content. Thus, with a 3-4OC temperature rise, the rice will drop 5
percentage points of moisture. From this point on, the drying rate changed
from 0.9- 1.2 percentage points per hour until 13:00, when the rate changed
t o 0.6-0.8 percentage points per hour. This lasted until either the sun set or
the 1 1 .5% moisture level. A t this 1 1 .5% moisture level, the rate of drying
changed once more t o 0.2 percentage points per hour. Final moisture contents
for the 6 tests on a 1 c m deep layer ran from 1 1 - 1 3.8%.
Concrete Surface
The concrete slab used in these tests was 8 c m thick and flat. No paint
was applied t o the surface. For this test a hole was drilled into the concrete
and a thermometer stuck tip first into the concrete. The other grain thermometers were again placed at the bottom and top of the rice layer which was
4 c m thick for each of the tests.
The concrete temperature maintained a fairly close range of 38.5-41 OC
and was found t o be always higher in temperature than the lower level of rice.
Generally it was 1 % - 2 C higher in temperature, though a t times it would
have the same value in the middle of the day. The temperature of both the
concrete and lower level of rice varied in a similar manner as the ambient
temperature, but within a closer range. The ambient temperatures were the
same as those in the matt drying tests and varied from 29 OC t o 36 OC within a
day's cycle.
Because of the high conductivity of the stone compared t o the loosely
packed rice, heat conduction away from the rice t o the ground does happen.
However, a basic heat balance across the concrete slab shows that heat loss
by conduction amounts t o less than 1 % of the incoming solar energy.
The top of the grain temperature varied from 34-46OC or 3-6OC higher
than the lower part of the rice layer. Temperature differences between the top
of the grain and the ambient air ran from 5OC during the morning to 13OC
around mid-day.
Moisture content started at 2 2 % each time and dropped t o 1 7 % by
1 4 : 0 0 on an average day (though on a clear day it was accomplished by
1 2 : 30). A better indicator here for the 1 7 % moisture point was a 45OC top
temperature and a 39OC lower level temperature. Drying rates do not appear
to be constant and vary between 0.7-1.2 percentage points per hour until
the grain reaches 14.5% moisture. A t this point, the rate drops t o 0.6-0.8
percentage points per hour. However, it usually takes the entire day to reach
14.5% moisture, and it is difficult t o say whether or not an actual change in
drying rate occurs or whether the solar radiation intensity has simply slacked
off at the day's end. Final moisture contents at the end of the day for the 4 cm
layer ranged from 13.7- 14.9%.
Comparison
Except for the physical layout, the methods are quite similar and only four
characteristics need t o be compared to understand their differences. (1), The
volume of the concrete surface method is four times that of the matt method,
yet, (2), only the temperatures under the rnatt and in the concrete differ
significantly. The third, (3), characteristic is that the matt drying shows all
three textbook stages of drying, (initial, falling and leveling out), while the
concrete surface method operates in only the first t w o stages where most of
the water is lost. Thus the matt drying method shows a greater volume of
water lost per volume of rice. However, the optimum moisture content is
1 3 - 14%, and it is not optimum to dry the rice t o the extent that the matt
method does. In turn, the final moisture content levels of the concrete surface
method need t o be a little lower. This could possibly be accomplished by drying
more rice at one time with the matt method and less rice with the concrete
surface method.
The last, (4fh), characteristic is that of an overall efficiency. If we define
the efficiency as the energy used to vaporize the water, divided by the solar
energy that falls on the rice surface, we can determine the overall performance
of each method. Using this definition, the efficiency for the bamboo matt
method is approximately 8%, while the concrete surface method has an
approximate efficiency of 2 6 % .
If all of the rice had dried at the same performance level as the matt
method, we would have expected the concrete surface method's efficiency to
be 4 x 8 % or 32%. However, because there was a difference in the final
moisture contents of the t w o methods, the concrete surface method had a
lower efficiency. Thus, if the height of the grain on the concrete surface is
decreased, we expect the moisture content to decrease as well. This suggests
that an optimum efficiency for the matt drying method should be approximately 7.5% while 7.5 x 3 = 22.5% is best for the concrete surface
method.
SOLAR WATER DISTILLATION UNIT
About a year ago we built a solar water distillation unit at UNSVIAH, and
though it has littie to do with rice drying, some of the results cast some light on
the concrete surface method. The unit consists of a concrete slab of about
8 em thickness, with short wails built around it. Two opposing walls are sloped
upwards towards- their centers at a 1O0 slope, and support beams placed
across their centers. Glass windows were then placed over the entire unit, and
the concrete was painted black.
Operation is simple. Water is placed in the unit, which vaporizes in the
heat of the sun. The vapor then rises and condenses when it touches the
windows. From there it flows down the bottom side of the window to pipes
which carry it outside of the unit.
General efficiency of the unit is about 25%. Temperatures of the water rise
to approximately 63OC by 3 3 : 0 0 and the concrete temperatures average
1 - 5 O C higher than the water temperature, while the glass temperatures run
1 -5OC cooler. Average output is around 1.75 I/m2/dayor 2 0 literlday for the
unit.
A heat up time is necessary before vapor is produced each day and
generally it is 11 : 00 before the first liter is produced. After 1 1 : 0 0 water is
distilled at the rate of 2 - 3 liters per hour until dark (19 : 00 in Aceh) and
continues on through the night until all of the usable energy in the concrete
slab is exhausted. Approximately 113 - 112 of the unit's output is produced
after the sun has gone down.
Looking only at the concrete, we see that because of the black color alone
the temperature runs about 15O higher than the top surface temperature of
the rice surface mentioned in the traditional methods discussion. Approximately 4 0 % of the usable energy was stored in the concrete to be used after
the sun had set. This is a solar passive design concept that is used by
architects to store heat in a building for winter heating. Here, there is no
winter, but the same concepts should be applicable in future modifications of
the traditional solar rice drying methods in Indonesia.
A.I.T.
SOLAR RICE
DRYER
A t this time we are in the process of constructing a solar rice dryer
designed by A.I.T. in Bangkok. This unit is made chiefly of bamboo poles and
plastic and consists of a solar collector area with an adjoining room containing
a drying table. The following figure shows the basic layout. From points AA to
BB is the solar 'collector (which is nothing more than an earthen area covered
over with plastic). Air heats up in this area and rises naturally under the plastic
to the bottom of the table, at point C. From there it passes through the rice on
the table and up out of the chimney at point E. Rice is placed on top of the table
by opening the doors at D and simply pouring it out evenly. .
Fig. 2.
A.I.T. Solar Rice Dryer.
The solar collection area for the unit is 4.5 m x 7 m, and the drying table
area is 1.5 m x 7 meters. Unit capacity is one ton or 1.6 m3 of rice, and the
average time to dry the rice from 22% moisture t o 13% is 2 days.
We have selected this unit because it fits both into our solar program as a
solar demonstration experiment and because we wanted to demonstrate a rice
dryer of medium capacity. As far as performance is concerned, there is no
advantage to this unit. The traditional concrete surface method takes the same
drying area for one ton of rice as this unit does, and an operator will usually dry
for only one day.
Characteristics of handeling and uniformity of drying, however, are much
more important with cracking duk to dryness greatly reduced and the chance
of re-wetting from a sudden rain storm eliminated. The rice is also up off of the
ground so that extra dirt from the atmosphere can not settle on the rice, thus
improving the cleanliness of the rice.
The cost of the unit, given by A.I.T., is approximately Rp 200,000.which, at this point in time, is too expensive for most farmers. We are
expecting, however, the demand for not only higher quality rice, but higher
quality agricultural products in general, t o increase in the next ten years. Thus
in turn, creating a demand for this unit.
After preliminary studies on rice drying, we will test the unit for clove
drying. This is a high cash crop for richer urban dwellers who own property in
the country. These people should be willing t o not only purchase such a unit
for their cloves, but also be willing t o use it for rice drying on their rural family
property. In this manner, we hope the unit will be accepted in Aceh over a
10- 15 year period.
FUTURE RESEARCH
Future research in solar rice drying will have t o focus on specific aspects
of both solar radiation energy absorption by the rice and heat storage in the
drying system before serious alternative designs can be made. The use of
either a black surface below the rice or a black plastic sheet on top of the rice,
for example, may increase the heat properties by 5- 10% while storage of
heat in the concrete could provide even higher efficiencies.
Some ideas that I personally have are as follows, (1) Painting the concrete
black for concrete surface drying, ( 2 ) Using a strip of black plastic with holes
punctured in it on top of the rice t o increase the amount of energy absorbed
into the rice, (3)Altering the A.I.T. design so as t o use a black concrete
surface as the solar collection area, and (4) Building a water distillation - salt
production unit that could readily convert to a grain dryer. Determining
whether or not an idea is realistic is the function of a research program, and in
the future, we at UNSYlAH hope that some of our ideas can realistically add to
the performance of the solar rice drying done in Aceh.